This invention relates to the growth of diamond clusters.
The use of seeds to control crystallisation by controlling the number of nucleation sites is well known in the art of crystal growing. In the case of diamond crystal synthesis, small diamond particles may be used as seeds to promote the domination of crystal growth on the seeds rather than crystal growth by spontaneous nucleation. For such applications, it is desirable to ensure that the seeds have a known size distribution so that numbers of seeds can be controlled, and that the seeds are distributed evenly and discretely.
Generally, in the art of growing diamond crystals by high pressure, high temperature (HPHT) synthesis, the seeds are diamond particles which are non-twinned, single crystals which are selected on the basis of size alone. Such seeds are usually made by crushing larger HPHT synthetic diamond crystals, and the diamonds grown using these seeds are dominated overwhelmingly by non-twinned, single crystals with a cubo-octahedral morphology. In this method of growing diamond crystals, the difference in solubility between graphite and diamond under substantially the same conditions of pressure and temperature is used as the driving force (supersaturation) for crystallisation. This method is otherwise known as the allotropic change method.
In the particular case of the growth of large single crystal diamonds, the seeds are generally somewhat larger to allow the seeds to be oriented crystallographically, and thus promote the growth of the diamond in a preferred crystallographic direction. In the special case of the growth of single crystal diamonds with a plate habit, seeds with macroscopic multiple twin planes are selected and appropriately oriented to allow crystal growth to occur in the preferred crystallographic direction as taught by European Patent Publication No. 0 780 153 (1997). In these methods of growing diamond crystals, the difference in solubility between diamond at two different temperatures and substantially the same pressure is used as the driving force for crystallisation. This method is otherwise known as the temperature gradient method.
According to the present invention, a diamond cluster comprises a core and an overgrown region containing a plurality of diamond crystallites extending outwards from the core, the majority of the crystallites having a cross-sectional area which increases as the distance of the crystallite from the core increases. Generally, at least 80% of the crystallites have a cross-sectional area which increases as the distance of the crystallite from the core increases.
The diamond crystallites will generally have a low concentration of inclusions such as metal inclusions and preferably less than 1% mass of inclusions.
The external surfaces of the diamond crystallites will generally be well defined crystallographically surfaces.
The core preferably comprises a bonded mass of constituent diamond particles.
The size of the diamond clusters which are crystalline can vary over a wide range, but will typically have a size in the range 50 microns to 1 mm.
Further according to the present invention, a method of producing a plurality of diamond clusters includes the steps of providing a source of carbon, providing a plurality of growth center particles, each comprising a bonded mass of constituent particles, producing a reaction mass by bringing the carbon source and growth center particles into contact with a solvent/catalyst, subjecting the reaction mass to conditions of elevated temperature and pressure suitable for crystal growth, and recovering a plurality of diamond clusters from the reaction mass.
The, growth center particle will provide a number of randomly oriented nucleation sites by virtue of its structure and the initial crystals that grow will exhibit a variety of crystallographic directions depending upon the growth center""s structure. Some of these crystals will be oriented so that they grow in the fastest growing direction, whilst other crystals will grow more slowly. Depending upon the number of nucleating sites in the growth center, the degree of interference of adjacent growing crystals and their growth directions, the growth of some crystals will be terminated early whilst others will continue growing. This will result in a crystal cluster whose structure is related to the structure of the original growth center particle. Furthermore, when the constituent particles comprising the growth center particle have multiple twin planes, the resultant grown crystal cluster will comprise crystallographically twinned crystals. Moreover, the twinning structure of the growth center particle may contribute to faster growth in particular crystallographic directions and so play a role in the selection of terminated crystals and those that continue to grow.
Thus, it has been found that the method of the invention produces clusters of diamond crystals, with the number of crystals comprising the cluster ranging from a few crystals (less than ten) to several hundreds of crystals. The crystals are generally substantially faceted and the clusters are substantially free of solvent/catalyst. Such clusters may be made up predominantly of single crystals, or predominantly of twinned crystals.
It is possible by appropriate selection of the growth center particles to produce clusters of selected and controlled or tailored structure. These clusters may be used in abrasive particle applications such as grinding, sawing, cutting, turning, milling, boring or polishing.